Scientists have produced the clearest picture yet of a key indicator of global warming - the world’s shrinking polar ice sheets. The international study ends a 20-year dispute over how much ice is being lost or gained in Greenland and Antarctica.

Measuring ice sheets is a tricky business. Over the last two decades, dozens of studies on ice loss have come up with conflicting results. Now, a global scientific team, reporting in the journal Science, has tried to set the record straight.

Different Assessment Methods Agree

Andrew Shepherd is a professor of Earth sciences at the University of Leeds in England and lead author of the study. He says teams of experts from 25 international institutions, working at the same time and in the same locations, used different methods to gather ice-loss data, and then compared what they found.

“When you look at the data, the ice sheets do have their own weather," he said. "From month to month we can see very different amounts of ice in Antarctica as compared to Greenland for example, and it becomes very important when you are comparing different estimates, that they’re made over the same time period and over the same geographical area.”

Shepherd says the results from the diverse research teams are two to three times more accurate than the most recent report from the Intergovernmental Panel on Climate Change (IPCC), the scientific body set up by the United Nations to regularly assess climate change.

“The total amounts of ice lost from Greenland and on Antarctica all together since 1992, amounts to about 11 millimeters of global sea level rise,” Shepherd said. Of that 11 millimeters, he adds, two-thirds to three-quarters is from Greenland and the remainder is from Antarctica.

All Ice Sheets Losing Mass Except East Antarctica

While 11 millimeters might not seem like much, Shepherd says the combined rate of ice melt has increased over time.

“In the 1990s the ice sheets were losing about 0.3 millimeters of ice in terms of global sea level equivalent each year," he said. "Right now they are losing three times as much.”

The study finds that all ice sheets are losing mass - except for East Antarctica, where climate change is having a different effect.

“In East Antarctica, "Shephard noted, "it’s a very large area and receives a very large amount of snowfall. The signal of growth is consistent with that story.”

New Assessment Critical to Climate Modeling

Co-author Ian Joughin, a glaciologist at the University of Washington, says the information they’ve gathered for the study is critical to modeling global climate and predicting sea level rise. He says even tiny changes in sea level, when added over an entire ocean, can have a substantial impact on storm surge and flooding forecasts for coastal and island communities.

“It’s important to keep in mind," Joughin says, "that this is a snapshot of the current mass balance of the last two decades’ mass balance of the ice sheets.” The expected warming, he predicts, will accelerate losses going further into the rest of the century.

Lead author Andrew Shepherd says he hopes the international teams will continue their ice-sheet surveys each year, and fill in some of the gaps left by less-frequent IPCC climate assessments.

“We hope that this is an annual exercise where every team that has contributed to this assessment can do this on a more regular time than the IPCC reports, which are very useful assessments on a five- or six-year cycle, but in the intervening five or six years, it leaves people in the dark as to how things have changed,” he said.

The new ice-sheet data, reported in the journal Science, comes amid a flurry of new climate-change reports published in recent days, while international delegates in Doha, Qatar, discuss a new U.N. climate change treaty. Those talks conclude on December 7.